This invention relates to test instrumentation generally and more specifically to instrumentation for testing cables for ADSL (Asymmetric Digital Subscriber Line) application, such as full rate ADSL and G.Lite ADSL.
A number of technologies have emerged to make better use of the bandwidth available on existing copper access networks. One of these technologies is xDSL (any of various types of Digital Subscriber Line). The major advantage of high-speed xDSL technologies is that they can all be supported on ordinary copper telephone cables already installed in most commercial and residential buildings. The most promising technology of the xDSL family is ADSL (Asynmmetric Digital Subscriber Line). Full rate ADSL provides downstream data rates of up to 8 Mbps (million bits per second) and upstream data rates of up to 1 Mbps. A subset of full rate standard is G.Lite ADSL, which provides downstream data rates of up to 1 Mbps and upstream data rates of up to 512 Kbps (thousand bits per second) depending on the telephone line condition.
Cable Qualification Test and ADSL Modem Test are two complementary tests required for deploying and troubleshooting ADSL service over the existing copper lines which were originally designed for voice service. Cable Qualification Test is to verify and troubleshoot a cable for an ADSL service by detecting and measuring any impairment of the cable. ADSL Modem Test is to verify the data rate of a cable and to troubleshoot an ADSL service with a data rate that is lower than expected. Without completing a successful ADSL Modem Test, service providers can not estimate the practical data rates for a potential ADSL service by just doing a Cable Qualification Test. On the other hand, a service technician can not tell from a failed ADSL Modem Test what caused the failure unless he or she can carry out a comprehensive Cable Qualification Test.
DMT (Discrete Multi-Tone) modulation is the main technique employed by ADSL modems. It is the standard modulation adopted for use in ADSL systems by ANSI (American National Standards Institute), ETSI (European Telecom Standards Institute), and ITU (International Telecommunications Union). A pair of ADSL DMT modems will operate in Frequency Division Multiplexing (FDM) or Echo Cancellation Mode by dividing the available frequency bandwidth into up to 256 sub-channels, or tones. Each sub-channel is modulated using QAM (Quadrature Amplitude Modulation) and carries between 0 to a maximum of 15 bits/symbol/Hz. The number of bits assigned to each sub-channel is based on the measured Signal to Noise Ratio (SNR) of the cable within the sub-channel. Also, each sub-channel""s data rate may be dynamically adjusted to adapt to the varying telephone line characteristics. The overall downstream and upstream data rates of the cable will be the total data rates of those sub-channels allocated for the downstream and upstream respectively.
A general connection for an ADSL modem test of a local loop is shown in FIG. 1, where the test unit labeled as ATU-C (ADSL Central Site Terminal Unit; or ADSL Transceiver Unit, Central Office End) is acting as the ADSL modem at the Central Office (CO) and the other one labeled as ATU-R (ADSL Remote Terminal Unit; or ADSL Transceiver Unit, Remote Terminal End) is acting as the ADSL modem at a customer""s premises. U-C and U-R stand for the loop interfaces at the CO site and at the remote terminal or customer""s premises site. Typically, when doing a field service, one of the test units can be the ADSL device or equipment installed at the CO or at the customer""s premises.
The present invention is to provide a powerful and unique tool to estimate the theoretical and practical data rates of a cable for various modem parameters without doing different ADSL modem tests with different sets of ADSL modems. The present DMT test can also be used to analyze the causes of, or to predict a problem respecting, the failure of a pair of ADSL DMT modems to synchronize.
The invention relates to a DMT (Discrete Multi-Tone) test method. To estimate the theoretical data rates of a cable without employing any ADSL modem, the method measures the frequency characteristics of the cable with discrete tones and adjusts the optimum AGC (Automatic Gain Control) setting for each tone. To estimate the practical data rates of a cable without employing any ADSL modem, the method measures the frequency characteristics of the cable with discrete tones and determines the data rate based on the ADSL DMT standard and modem parameters selected by the user. By synchronizing at lower frequencies, such as voice band frequencies, the method provides a means to predict any possible failure of a potential ADSL service and/or an ADSL modem test on a telephone cable when an ADSL modem test would fail with the cable.
In a major aspect, the invention comprises a method for performing a DMT test to determine cable data rates using two. test devices, without employing ADSL modems, on a telephone cable or a local loop cable for ADSL application, whereby the available frequency bandwidth is divided into sub-channels, the method comprising the steps of synchronizing a transmitter and receiver in each of the two test devices at lower frequency signals, measuring the frequency characteristics of the cable with test signals, measuring cable noise and interference for every sub-channel, determining a theoretical and practical data rate for every sub-channel based on the measured frequency and noise characteristics, and modem parameters selected by the user, and determining the theoretical and practical data rates for the ADSL bandwidth based on an ADSL DMT standard selected by the user.
In additional aspects of the invention:
(a) the test signals and cable noise are converted into digital form at the receivers for processing;
(b) coded or modulated signals are employed for passing information between the two test devices for test parameters, such as ADSL standard and selected modem parameters, and test results;
(c) the step of synchronizing the two test devices at lower frequencies comprises the steps of synchronizing two test devices in a tone, or a modulated signal, of voice band frequencies, sending a synchronization signal at up to the maximum power allowable for the cable under test, and adjusting the AGC setting of the receiver for synchronization signal;
(d) the step of synchronizing the two test devices at lower frequencies comprises the steps of synchronizing two test devices in a tone, or a modulated signal of frequencies close to voice band, or lower frequencies in the ADSL frequency band, sending a synchronization signal at up to the maximum frequency allowable for the cable under test, and adjusting the AGC setting of the receiver for the synchronization signal;
(e) a time-domain, or frequency-domain analysis method is used for detecting and receiving the synchronization signal;
(f) the step of measuring the frequency characteristics of the cable with discrete tones comprises the steps of sending individual test tones, one tone at a time, at up to the maximum power allowable for the cable under test, adjusting the AGC setting of the receivers for each sub-channel accordingly, measuring the attenuation characteristics of the cable for each sub-channel, and calculating the frequency characteristics of the cable for each sub-channel;
(g) the step of measuring the frequency characteristics of the cable with discrete tones comprises the steps of sending individual test tones, a few tones at a time, at up to the maximum power allowable for the cable under test, adjusting the AGC setting of the receivers for the few sub-channels accordingly, measuring the attenuation characteristics of the cable for each sub-channel, and calculating the frequency characteristics of the cable for each sub-channel;
(h) a time-domain or frequency-domain analysis method is used for detecting and receiving the test signals;
(i) the step of measuring the cable noise and interference for every sub-channel comprises the steps of measuring the level or energy level of the cable noise and interference for sub-channels of upstream and downstream respectively at the two test devices connected for the test, and adjusting the AGC setting of the receivers when required;
(j) the step of measuring the cable noise and interference for every sub-channel comprises the steps of measuring the level or energy level of the cable noise and interference for all sub-channels of the whole ADSL bandwidth at the one or two test devices connected for the test, and adjusting the AGC setting of the receivers when required;
(k) a time-domain or frequency-domain analysis method is used for measuring the cable noise and interference;
(l) the step of determining the theoretical and practical data rates for every sub-channel based on the measured frequency and noise characteristics, and modem parameters selected by the user comprises the steps of converting the measured signal level at the receiver input of the test devices to the signal level of an ADSL modem receiver for each sub-channel, calculating the optimum signal-to-noise ratio (SNRo) of each sub-channel, calculating the practical signal-to-noise ratio (SNRp) of each sub-channel according to the ADC (Analog to Digital Converter) resolution selected by the user for the target ADSL modem, determining the theoretical data rates from the calculated SNRo. for each sub-channel for the modem parameters selected by the user, and determining the practical data rates from the calculated SNRp for every sub-channel for the modem parameters selected by the user;
(m) the step of determining the theoretical and practical data rates for the ADSL bandwidth based on an ADSL DMT standard selected by the user comprises the steps of estimating the theoretical and practical data rates of upstream at U-C according to the ADSL DMT standard selected by the user, estimating the theoretical and practical data rates of downstream at U-R according to the ADSL DMT standard selected by the user, and estimating the theoretical and practical data rates of the whole ADSL bandwidth according to the ADSL DMT standard selected by the user;
(n) the step of determining the theoretical and practical data rates for the ADSL bandwidth based on an ADSL DMT standard selected by the user comprises the step of estimating the theoretical and practical data rates of upstream and downstream at U-C or U-R or both sides according to the ADSL DMT standard selected by the user;
(o) the steps of determining a theoretical and practical data rate for every sub-channel based on the measured frequency and noise characteristics, and modem parameters selected by the user, and determining the theoretical and practical data rates for the ADSL bandwidth based on an ADSL DMT standard selected by the user, further include predicting any possible failure of a potential ADSL service and/or an ADSL modem test on a cable when a pair of ADSL modems would fail to synchronize over it;
(p) the steps of determining a theoretical and practical data rate for every sub-channel based on the measured frequency and noise characteristics, and modem parameters selected by the user, and determining the theoretical and practical data rates for the ADSL bandwidth based on an ADSL DMT standard selected by the user further include analysing the failure of an ADSL service and/or an ADSL modem test by showing, in text or graphic format, the converted measured signal levels, measured cable noise levels, and/or the bits allocated, for each sub-channel of ADSL bandwidth or for those sub-channels used by a pair of ADSL modems for synchronization purposes.
Further aspects of the invention will become apparent from the description of an illustrative embodiment which follows.